1. Field of the Invention
This invention relates to a feeble light measuring device of photon counting type for use in streak camera systems, feeble light imaging devices and so on.
2. Related Background Art
FIG. 1 shows a block diagram of the conventional photon counting type-streak camera system. This conventional streak camera system comprises a streak tube 101, a driving circuit 102 for driving the streak tube 101, a highly sensitive television camera for detecting a streaking image formed on a phosphor screen 101a of the streak tube 101 through a relay lens 104, and an image processing system 106 for reproducing, storing and processing video signals from the highly sensitive video camera 105. The streak tube 101 comprises an photocathode 101e, an acceleration electrode 101b for accelerating photoelectrons, a deflecting electrode 101c for scanning accelerated photoelectrons, and microchannel plate (MCP) means 101d provided by a pair of MCPs for multiplying the scanned photoelectrons.
Light to be measured is converted into a streaking image by the streak tube 101. The streaking image is converted into video signals by a highly sensitive camera 105. The video signals are A/D converted by the image processing system 106 to be integrated as image data for the improvement of S/N ratios of the signals. The vertical axis of the image data obtained by the image processing system 106 is corresponding to the time axis and the horizontal axis of it is corresponding to the spacial axis on the photocathode 101e. An intensity value of the image data corresponds to a light intensity of the light to be measured. The image processing system 106 generates transient waveforms (time-dependent radiation) of the light to be measured from the image data, and can further display and analyze the transient waveforms.
The conventional streak camera system is disclosed in "High Speed Photography, Videography, and Photonics IV (1986)" SPIE, vol. 693, p. 99, or Japanese Patent Application Unexamined Publication No. 58745/1984.
However, in the conventional streak camera system, the MCP means 101d provided on the streak tube 101 is operated in its saturated state corresponding to the incidence of a single photoelectron, and it is difficult to measure very feeble light events easily with high precision. Specifically, in this case, by superposing two microchannel plates, a single photoelectron is multiplied by about 10.sup.6 to saturate the MCP means. Consequently due to spread of the multiplied electrons, the sensing precision of an incident position of the photon is degraded. Such resolution decrease can be solved to some extent by giving a barycenter of a spread luminescent spot. But problems with such solution are that the barycenter detection of the luminescent spot needs complicated computation, and takes remarkably much processing time. In addition, it decreases the production yield of the streak tube and increases costs to incorporate two MCPs in the streak tube. When the defect ratio of the streak tube including one MCP is 30%, the defect ratio of the streak tube with two MCPs is as high as 50%. This is also a problem in providing inexpensive streak camera systems. A further problem is that the use of the highly sensitive television camera 106 for taking an image formed by the streak tube makes the system large.
FIG. 2 shows a conventional example of the feeble light imaging device using an image intensifier and a CCD camera. An image to be measured is imaged on a photocathode 202a of an image conversion tube 202 by an objective lens 201. Photoelectrons proportional to an intensity of the image are emitted from the photocathode 202a and are incident on an MCP 202b. The photoelectrons incident on the MCP 202b are multiplied by about 10.sup.4 by an MCP 202b and are incident on a phosphor screen 202c to be converted again into an optical image. An output image of the phosphor surface 202c is led to a fiber plate 203 and is incident on a CCD 204 to be converted into electric signals. Output signals of the CCD 204 are intensified by an image intensifier 205 and A/D converted to be inputted to an adder. The adder reads the image data stored in an image memory up to the immediately preceding operation, and adds the inputted signals thereto. The addition result is stored as new data in the image memory.
The art of repeating such addition operation to thereby improve the S/N ratio of a sensed image is well known, and the simplicity of the art makes it prevalent. In this case, additions increase the signals, but dark current noise of the CCD image sensor, and video read out noises generated by the CCD and Amplifier are also increased by additions. A problem with this art is that the sensing of a feeble light image in a single photoelectron counting region is difficult.
As a method for obtaining an image of a feeble light event in a single photoelectron counting region which cannot be sensed by the above-described feeble light imaging device, a photon counting method is known. The photon counting method multiplies by 10.sup.6 or more a single photoelectron corresponding to a light event by an electron multiplier comprising a multi-stage MCPs, exactly separating noises generated in an image intensifier and noises generated in reading a CCD from signals indicative of a single photoelectron corresponding to a feeble light event, and detecting only the signal components with high sensitivity.
FIG. 3 shows a feeble light imaging device using this photon counting method. The device of FIG. 3 uses an MCP device including a multi-stage MCPs 202d, 202e connected in a cascade for multiplying a single photoelectron by 10.sup.6 or more. The MCP are connected to each other with a gap therebetween, and output electrons of one channel of the first stage MCP 202d are inputted to a plurality of channels (usually 30 or more) of the second stage and its following MCPs 202e. Consequently the output responding to a single photoelectron on the final stage is a synthesized output of groups of electrons from the plural channels, and its luminescent spot on the phosphor screen 202c is spread. The second and its following MCPs 202e are used in saturating states, therefor MCP 202e operates in ranges having a saturating number of output electrons for any number of input electrons, and accordingly the luminescent spot on the phosphor screen has a blunt luminance distribution. Consequently the resolution is much lowered. FIG. 4 is a view illustrating a luminescent spot generated on the phosphor screen by the multiplication of a single photoelectron. FIG. 4A illustrates the incidence of groups of multiplied electrons on the photocathode 202a. FIG. 4B illustrates a distribution of luminance intensities on the phosphor screen 202c.
To improve such deterioration of the resolution, in the feeble light imaging device of FIG. 3, the barycenter detecting function is provided before the image memory for the computation of a luminescent spot, and the computation result is supplied to the adder and stored in the image memory. In detecting a barycenter, reading noises (noises generated by the CCD and the image intensifying circuit) are completely removed and does not increase during a long period of time of the computation.
This feeble light imaging device of FIG. 3 is a good method for sensing a feeble light image in the single photoelectron region, but has the following problems.
1) The use of a multi-stage MCPs lowers nondefect ratios of the image conversion tube, and costs of the image conversion tube go up. Each MCP includes a bundle of, e.g., 3,000,000 or more glass tubes, which tend to be stuffed or have defects of discharges. A multi-stage MCPs have defect ratios which are deteriorated by a multiple of a number of the MCPs, which results in higher costs.
2) The barycenter center detecting function is necessary. The usual read rate of TV camera is 30 frames per minute and the barycenter center detecting function needs considerably high-speed computing ability. Such function is very expensive.
3) An output image of a single photoelectron is so large that counting rate is low, and some cases sufficient resolving power cannot be obtained. For example, when two light points on the photocathode come gradually nearer to each other, the two points cannot separate from each other at a position where the outputs images begin overlapping each other and are adversely sensed as one point. Accordingly in cases of considerably large quantities of light, or partially bright events, the measuring precision is conspicuously deteriorated. FIG. 5 shows views explaining the deterioration of the resolving power. As shown in FIG. 5A, in the case that Points P.sub.1 and P.sub.2 are sufficiently separated from each other, images formed by a par of photoelectrons incident on Points P.sub.1 and P.sub.2 does not overlap each other on the phosphor screen 202c, and barycenters B.sub.1 and B.sub.2 corresponding the events can be independently observed. However, as shown in FIG. 5B, in the case that Points P.sub.1 and P.sub.2 come nearer to each other, images formed at Points P.sub.1 and P.sub.2 partially overlap each other both on the phosphor screen and on the photocathode, the result of the barycenter center computation is one point B at the center.